1. Field of the Invention
This invention relates generally to a process for depositing aluminum alloys into vias and trenches in semiconductor devices and more specifically, this invention relates to a low temperature process for depositing an aluminum alloy into sub-0.25 .mu.m vias and trenches. Even more specifically, this invention relates to process for depositing an aluminum alloy into sub-0.25 .mu.m vias and trenches with an Al--Ge--Cu alloy that has a reflow temperature less than 400.degree. C. and has excellent electromigration characteristics.
2. Discussion of the Related Art
The semiconductor industry is characterized by the ongoing requirement to increase the device integration in a semiconductor chip. This means that the packing density of the devices on a semiconductor chip has to be increased. To achieve increased packing density on a semiconductor chip, the device dimensions have to be decreased. Presently, there is an intense effort to reliably decrease the device dimensions below the sub-0.25 .mu.m dimension. A major part of this effort is concentrated in the area of improving the technology relating to contact hole or via filling, trench filling and the associated planarization technology. These areas of technology must be improved in order to achieve device reliability. As an example of the products that use such device dimensions, the 256 Mbit dynamic random access memory have contact holes which are 0.25 .mu.m in diameter and have an aspect ratio of 4. Conventional sputtering has proven unreliable in handling such a high aspect ratio because of its poor step coverage. Solutions involving blanket low pressure chemical vapor deposition (LPCVD) tungsten and selective LPCVD tungsten have been proposed and contact holes with 0.2 .mu.m diameter and an aspect ration of 3 have been successfully filled. However, tungsten has high resistance, high film stress, and is difficult to process.
Aluminum (Al) has a lower resistivity and a lower film stress than tungsten. Kuniko Kikuta and Takamaro Kikkawa, in an article entitled "Al--Ge Reflow Sputtering for Submicron Contact Hole Filling," Journal of the Electrochemical Society, Vol. 143, No. 1, January 1996, state that some researchers have attempted to overcome the problem of poor step coverage for conventional sputtering by using aluminum reflow technology, some applying it with laser irradiation and some with high substrate heating over 500.degree. C. However, the high temperature at which conventional Al alloys flow degrades the reliability of underlying metallization because of thermal stress-induced void formation. In this paper, Kikuta and Kikkawa proposed an aluminum-germanium alloy, Al--5% Ge reflow sputtering for submicron contact hole filling. Although Kikuta and Kikkawa obtained a substantial reduction of the reflow temperature to approximately 300.degree. C., the resistance of the Al alloy increased to an unacceptable level.
In a later paper, Kuniko Kikuta, Yoshihiro Hayashi, Tutomu Nakajima, Keiichi Harashima and Takamaro Kikkawa, in an article entitled "Aluminum-Germanium-Copper Multilevel Damascene Process Using Low-Temperature Reflow Sputtering and Chemical Mechanical Polishing," IEEE Transactions On Electron Devices, Vol. 43, No. 5, May 1996, describe a low-temperature multilevel aluminum-germanium-copper damascene technology that was developed using reflow sputtering and chemical mechanical polishing. The described process reduced the maximum processing temperature for the fabrication of multilevel interconnections to approximately 420.degree. C. using an Al-1% Ge-0.5% Cu. In a paper entitled "Electromigration Characteristics for Al--Ge--Cu," Journal of Electrochemical Soc. Vol. 143, No. 3, March 1996, Kikuta and Kikkawa describe their investigation of the electromigration characteristics of the Al--Ge--Cu alloy. They indicate that the use of an Al-5% Ge alloy achieved a low temperature reflow sputtering down to approximately 300.degree. C., but as described above, the high percentage of germanium increased the resistivity of the aluminum alloy to an unacceptable level. They conclude that an alloy of Al-1% Ge-0.5% Cu is suitable for reflow sputtering since the reflow temperature is approximately 400.degree. C.
However, because the process to manufacture a sub-0.25 .mu.m technology product that uses low-k dielectrics must have a maximum temperature below 400.degree. C. the processes and alloys described by the above articles do not meet the needs of the advancing semiconductor industry.
Therefore, what is needed is an aluminum alloy that has a reflow temperature less than 400.degree. C., has improved electromigration characteristics and has low resistivity.